In a still video system which records video signals on a magnetic disc, a rotational position detection signal from a rotating magnetic disc is used in a servo system for a magnetic disc drive motor and in the head selection for frame recording. The rotational position detection signal, or the so-called PG (pulse generator) signal, as illustrated in FIG. 9, is obtained when a PG yoke 2 provided on a magnetic disc 1 passes over a predetermined position (the position of a PG sensor 3) as the magnetic disc rotates.
One pulse of the PG signal is obtained every time the magnetic disc 1 makes one full turn. Specifically, the detection signal from the PG sensor 3 is waveform-shaped to obtain an APG signal (first PG signal), and then a square-pulse NPG signal (the second PG signal) is produced. Conventionally, the second PG signal has been used as the PG signal in the video signal recording.
The second PG signal is produced from the detection signal of the PG sensor 3 at the timing as illustrated in FIG. 10. An output (a) of the PG sensor 3 is first inverted to produce a signal (b). The inverted signals is then converted to a signal (c) by a zero-cross detector, and integrated into a signal (d). The integrated signal is converted to a first PG signal (e) by a hysteresis comparator, and integrated into a signal (f). This integrated signal is and converted to a second PG signal (g) by a hysteresis comparator.
However, the second PG signal (g) has a problem of position shift. For example, if a hum exists in a waveform shaping circuit when the second PG signal is produced, a second PG signal with a shifted phase is produced, which causes fluctuations in the second PG signal. This fluctuation causes an inconsistent length for 1 turn of the magnetic disc, which results in the recording of the video signal to be partially missing.